MERCER 9100 Series - Definitions of Pressure Relief Valve Terms

MERCER VALVE COMPANY, INC.
9100 SERIES DIRECT SPRING OPERATED PRESSURE RELIEF VALVE
INSTALLATION, OPERATION, AND MAINTENANCE MANUAL
MERCER VALVE CO., INC. RESERVES THE RIGHT TO CHANGE
PRODUCT DESIGNS AND SPECIFICATIONS WITHOUT NOTICE.
Rev.: J
Date: 23-July-2020
ECN: 3152 Page | 5
2. Definitions
Accumulation is the amount of pressure increase above the MAWP of the system that is allowed.
Accumulation is set from the applicable codes.
Backpressure is the amount of pressure on the outlet side of the pressure relief valve. This is
pressure downstream of the nozzle of the pressure relief valve. It is a combination of superimposed
and built-up backpressures.
Blowdown is the reclosing point of the pressure relief valve. It can be expressed as a pressure but
is most commonly expressed as a percentage of the set pressure.
Built-up Backpressure is pressure on the outlet of the pressure relief valve that is caused by the
flow of the pressure relief valve. Piping size, length of outlet pipe, fittings, or other flow friction
producing items all affect the built-up backpressure.
Capacity is the flow rate as required by the applicable code.
Cold Differential Test Pressure (CDTP) is the pressure at which the pressure relief valve is
adjusted to open on the test stand. The CDTP includes corrections for the service conditions of
backpressure and/ or temperature.
Conventional Pressure Relief Valve is a direct spring operated valve where the spring tension is
used to hold the disk closed. The operational characteristics of the valve are directly affected by
changes in the backpressure. These characteristics include opening pressure, closing pressure,
relieving capacity, and valve function. This is also referred to as a spring operated pressure relief
valve (SOPRV).
Inlet Size is the nominal pipe size (NPS) of the inlet of the pressure relief valve.
Leak Test Pressure is the pressure below the set pressure that the leak test is to be performed.
This pressure is often calculated as a percentage of the set pressure.
Lift is the amount of travel of the disk from the fully closed to the fully open position when the
valve is relieving.
Maximum Allowable Working Pressure (MAWP) is the pressure rating the system can
withstand at a specific temperature. The MAWP is often used as the setting of the pressure relief
valve.
Outlet Size is the nominal pipe size (NPS) of the outlet of the pressure relief valve.